Monitoring a 4-20mA signal

[geomit]

Does anyone know where I can find a quick circuit configuration for monitoring a 4-20mA signal either by a current viewing resistor and associated amplifier or some kind of pre-packaged IC.
Thank you.

 

[itsmoked]

Sure just stick a 499ohm resistor in series and you will have 10V across it for 20mA. Etc. Etc.

 

[OperaHouse]

Define monitoring. You can always use an opto isolator chip and use the current transfer function of the transistor. that would give a voltage burden of less than 2V. Not exactly linear, but if you are just "monitoring" should be good enough. There are loop powered meters that you can just stick in the line.

 

[felixc]

Itsmoked, I would a 49.9 ohm series resistor instead. Many 4-20mA circuits work from a 12V source, and a series resistor like 499 ohms will saturate the circuit, and fool the meaurements and the circuit being monitored as well.
Felixc

 

[itsmoked]

Actually felixc, I would use a 249 ohm to only consume 2.5 volts of head and match most micro inputs.

 

[analogkid2digitalman]

Why not just use a DMM with current function? Burden will be a measly 5-10mV per mA.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[itsmoked]

Bingo!

 

[ScottyUK]

Some time ago this subject came up in Eng-Tips, but like normal I can't find it! A nice solution was to insert a standard rectifier diode in the current path, and connect a mA meter across the diode when a reading was required. Provided the meter burden drops less than about 0.5V when passing the current, the diode isn't even thinking about conducting so the current measurement is accurate, and there is no requirement to break the loop for testing.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

Yes I have used that diode trick several times..
And yes I know what mean about searches!

I was just working on the latest, (once a week), 'I want my UPS to run for 100hrs', question.

I wrote a long discourse on the subject last week. I knew it had: "UPS acid battery" in it. I couldn't find it!